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Aspects of mUltiparous sno.... crab(Chionoecet:es opilio) fecundity in insular Ne....foundland waters
by
David M. Taylor B.Se.
Department of Biology Memorial University of Ne....foundland
A thesis submitted to the School of Graduate Studies in partial fulfilment of the requirements for the degree of
Master of Science
1996
St. John's Ne.... foundland
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ABSTRACT
The fecundity of mUltiparous snow crab (Chionoecetes opilio) females from eight Newfoundland snow crab management areas is reported. Specimens were obtained between 1983 and 1985 when exploitation of the snow crab resource was at a very high level. Three study areas were virgin, while five were heavily fished. Basic biological data such as size (carapace width) age (shell condition) and mating status (presence of new ejaculate in the spermathecae) of each individual were collected at the time of sampling. Gravid female size ranged from 44 to 85mm carapace width (mean 65.8 mrn). Fecundity was positively correlated with size ranging from 8,589 to 103,112 eggs (mean 44,658). Females from virgin areas had a higher size-specific fecundity than did those from exploited areas. Also, females utilizing new spermatophores had a higher size-specific fecundity than did those using old or a mixture of old+new spermatophores.
Physical factors such as depth or latitude did not appear to affect fecundity.
The accuracy of utiliZing external grasping marks as indicators of recent mating was investigated. The grasping mark status of females from three sources: nearshore time-
iii
series trapping surveys, offshore trapping and offshore trawling surveys was determined and correlated ....ith spermathecal condition as determined by dissection.
Grasping marks were found to berelatively reliable indicators of mating frequency/recency, correctly predicting spermathecal condition -70-80\: of the time. The proportion of new-shelled mUltiparous females that had recently mated was 1.5 times that of old-shelled multiparous females.
Fishery exploitation generally had little effect on the proportion of multiparous females bearing eggs but did have a highly significant negative impact on size-specific fecundity. Part of this effect is likely due to reduced mating frequency in exploited populations which leads to a greater reliance on stored sperm for egg fertilization.
iv
ACDlOWLBDGEXEltT8
I would like to thank Illy supervisor, Dr. Roy Knoechel, for his quidance and assistance throughout this study. His patience a.nd editorial expertise made the completion of this study possible.
I would also like to express my appreciation to members of my supervisory committee, Drs. J.Green and R.G. Hooper.
L.W. Coady, Regional Director of Science, Department of Fisheries and Oceans encouraged me throughout the course of this study and approved leave necessary tor its completion, while my good friend Dr. John Hoenig provided much appreciated encouragement.
I would also like to thank Or. Bernard sainte-Marie, Maurice Lamontaigne Institute, OFO, Mont-Joli, Quebec who graciously provided some unpUblished data and patiently explained some of the finer details of his work to me.
r would like to thank Paul O'Keefe and other technical staff of Shellfish Section who assisted in many aspects of the field work associated with this study. Or. R.G. Hooper provided much appreciated support and assistance in collecting Bonne Bay data. Gordon King provided photographic services while M. Rees assisted with typing.
Summer student Reid Piercy and Hurley Fisheries conSUlting, Dartmouth, Nova Scotia assisted in conducting the egg
counts. Paul Collins provided invaluable assistance in statistical analysis, computer preparation of figures and field activities.
I would also like to express my deep appreciation and love to my wife Luanne and my three sons Harry, Stewart and Alistair for their constant love and support throughout the course of this study and life itself.
Finally, I wish to dedicate this thesis to my grandmother, the late Jean M. Taylor who served as a primary school teacher in rural Cape Breton Island and Newfoundland for over 30 years. She instilled in me a love of nature, taught me the difference between a root and a rhizome but made me clean my own fish.
vi TABLEorCOHTEN'l'S Abstract . . . .
Acknowledgements Table of contents List of tables List of figures
Chapter 1: General Introduction
i i iv vi vii viii
Chapter 2:
Chapter 3:
Chapter 4:
References
Snow Crab Fecundity . . Introduction Methods • • . • • • . Results and Discussion Summary • • . • • • • External Classification of Mating Recency
Introduction . . . . . Methods . . . • . . . Results and Discussion Summary . . • . • General Discussion and Summary . .
12 12 13 23 46 47 47 51 58 70
71 81 Appendix A: Size-frequency distributions of male snow
crabs captured concomitantly with females for which fecundity data are presented . . . 94
vii LIST OF TABLES
Table 2.1: Relation of fecundity (Y number of eggs) to carapace width (X, mm) for several North American study areas . . . • . . . • . . . . 10 Table 2.2: Summary of the percentage of lIature females
bearing eggs on the commercial fishing grounds of 3 snow crab, C1:lionoecetesopilio, management areas 1982-92 . • . • . . . .25 Table 2.3: Summary of size-specific fecundity effects
on snow crab egg numbers and egg mass by sample area and fishery exploitation status. 29 Table 2.4: List of fecundity sample sites a.nd with
navigational coordinates . . . 33 Table 2.5: comparison of size-specific effects of shell
type on fecundity . . . 40 Table 2.6: Pairwise comparison of spermatophore type
effects on size specific fecundity in terms of egg number (upper right section) and egg mass (lower left section). Each cell contains the effect coefficient followed by its p value, sample size and calculated effect strength . 42 Table 2.7: Stepwise regression models of fecundity
versus carapace width and biological and site factors . . . 44 Table 3.1: comparison of efficacy of grasping marks as
indicators of mating recency for offshore trap and trawl-caught females. Numbers in brackets are 951; confidence intervals calculated for the binomial distribution. 61 Table 3.2: Summary of grasping marks and spermathecal
contents tor combined new and old-shelled offshore trap and tra....l-caught female sno....
crab . . . 63 Table 3.3: Grasping marks and spertllathecal contents
for trap-caught females from the nearshore Avalon PeninSUla, 1987-92 . . . 65
viii L:IS'l' OJ' J':IGtJJlES
Figure 1.1: Dorsal view of mature female snow crab Chionoecetes opilio. The larqer cleaner individual is new-shelled, while the smaller female covered with epibionts is old-shelled. Note the qraspinq marks on the leqs of both individuals. • . . . 3 Figure 1.1A: Ventral view of mature female snow crab
Chionoecetes opilio • . . . . 4 Figure 1.2: Newfoundland snow crab management areas,
circa 1985 . . . • • . . . . Figure 1. 3: Clutch of oranqe eggs attached to the
pleopods of a mature female snow crab Figure 2.1: sample sites where mature multiparous females were collected tor fecundity analysis: a)Bonne Bay, b)Foqo Island, c)Bonavista Bay, d) conception Bay, e) Nearshore Avalon, f) Southern Avalon, g) Offshore Avalon and h) Downing' 8&sin. .15 Figure 2.2: Scatter plot and regression of fecundity
(egg number) versus carapace width for females from all study areas combined. .27 Figure 2.3: Scatter plot of fecundity (eqqnumber)
versus carapace width for females from Downing Basin (solid figures) and Southern Avalon (open figures) and reqression line for all areas combined (n
=
350), equation in Table 2.1) . . . 31 Figure 2.4: Size-specific fecundity versuslatitude. . . 34 Figure 2.5: Size-specific fecundity versus depth. 36 Figure 2.6: Size-specific fecundity versus mean
size of concomitantly captured male snow
crab. 38
ix
Figure) .1: comparison of size-frequency distribution of concomitantly capture male sno.... crabs from a virgin area (Bonne Bay) and a heavily exploited area (conception Bay) . • 49 FigureJ.2: Spermathecae bearing neW' spermatophores
only . . . • . • . . . • . . . .53 Figure 3.3: Spermathecum bearing new+old spermatophores.
New spermatophores can be observed as a white matrix in the narrow part of the
structure while old dark coloured spermatophores are in the wider apical portion . . . 54 Figure]. 4: Legs of multiparous female depicting the
three types of qrasping marks. Uppermost leg bears new marks only, the middle leg bears old marks only while the lower leg bears both old and new marks • • • • • • 56 Figure 3.5: Location of offshore survey sites, 1989-92.
"NA" dnd large solid circle denote Nearshore Avalon time-series sampling site. Lower case letters and small solid circles denote offshore sampling sites: a) Funk Island, b) Offshore Avalon, c) offshore Avalon, d)Oowning Basin, e) Haddock Channel and f) Halibut Channel . . . 59 Figure 4.1: Size-fecundity regressions for female
snow crab from Newfoundland compared with other North American study areas: a) Anticosti Island, b) Bering Sea, c)Cape Breton, d) Gabarus , e)Gulf of st. Lawrence, f) Baie Sainte-Marqueri te, g) Newfoundland 1981, h) Pleasant Bay and i) present study . . . . 75 Figure AI: Size-frequency distribution of males from
Fogo Island caught concomitantly with females examined in this fecundity stUdy .95 Figure A2: Size-frequency distribution of males from
Bonavista Bay caught concomitantly with females examined in this fecundity stUdy .96
Figure AJ: Size-frequency distribution of :males trom Nearshore Avalon caught concomitantly with females examined in this fecundity study. 97 Figure A4: Size-frequency distribution of males trom
Offshore Avalon caught concomitantly with females examined in this fecundity study .98 Figure A5: Size-frequency distribution of males from
Downing Basin caught concomitantly with females examined in this fecundity study .99
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1-2
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Figure 1.1: Dorsal view of mature female snow crab Chionoecetes opilio. The larger cleaner individual is new-shelled, while the smaller female covered with epibionts is old-shelled. Note the grasping marks on the legs of both individuals.
Figure l.lA: Ventral view of mature female snow crab Chionoecetesopilio.
attempt to limit the snow crab harvest to within recommended exploitation rates of 50-60\ (Anon. 1981) many snow crab management areas in Newfoundland have experienced harvesting levels far in excess of these guidelines (Taylor and O'Keefe 1987). It has been hypothesized that excessively high exploitation rates might negatively impact snow crab reproductive potential if many primiparous females remain unmated barren following the pubertal molt due to a lack of large males (Conan and Comeau 1986). Although recent field and laboratory studies in Newfoundland have shown that sub- legal males are capable of mating with and fertilizing multiparous females (Ennis et al. 1988, 1990), laboratory and field studies on cogener c.bairdi in Alaska (Paul and Paul 1996 and Stevens et al. 1993) suggest that small males have difficuLty in copulating successfully with multiparous females. Therefore, the potential impact of removal of large males by commercial fisheries on the reproductive output of natural populations due to effects on individual fecundity should be evaluated.
Several snow crab management areas in NAFO Division 3L (Figure 1.2) experienced dramatic declines in landings and catch rates during the early 1980's (Taylor et al. 1994).
Catch rates fell to such 10.... levels (CPUE= kg/trap haul) between the fall of 1982 and the spring of 1983 that for all intents and purposes the fishery collapsed. In the Avalon
600 59" 58" 57" 54' 53' 52' 51' 50'
52'
·N· ~
~2°51'
I
51'50' ~OO
49°. 49°
48' 4S'
47' 47'
Figure 1.2: Newfoundland snow crab manaqement areas, circa1985.
Peninsula area (Management Areas 15, 18 and 19, Figure 1.2), fishermen virtually abandoned the fishing grounds between 1985 and 1988 due to the scarcity of commercially acceptable male snow crab (Taylor et al. 1994). This situation presented an opportunity to examine the reproductive status of mature females to determine whether the decline in large males for a period of several years had impacted either tht!
proportion of mature females that carried eggs or the number of eggs females carried.
aeproductive Bioloqy
Most researchers believe that female C. opilio and their congeners undergo a terminal molt to maturity in the winter or early spring when they are between 5-7 years old (Yoshida 1941; Ito 1963; Watson 1970, 1972; Hilsinger 1976).
This molt to maturity usually takes place with the assistance of a male partner while in the pre-copulatory embrace (Watson 1970, 1972; Hooper 1986; Donaldson and Adams 1989; Sainte-Harie and Hazel 1992). However, Ito and Kobayashi (1967) and Hooper (1986) have reported mature females (identif ied by the presence of a wide rounded abdomen) in the process of molting. Following copulation, females extrude bright orange eggs for the first time, and are termed "primiparous". Females that repeat spawn a second or third time in later years are referred to as
"multiparous". The eggs are attached to pleopods (Figure 1.3) that are present on the underside of the abdominal flap. The eggs become progressively darker during the incubation period as the eyespot enlarges during development. The eggs undergo an incubation period. ranging from one year (Watson 1972) to slightly more than t ....o years (Sainte-Marie 1993) before pre-zoeal stages are released in the spring immediately prior to, or just after re-mating (Taylor et a1. 1985; Hooper 1986).
Various aspects of femaleChionoecetes sp. fecundity have been studied in recent years. These studies and their findings are briefly summarized in Table 2.1. Sperm storage and the functional anatomy of the femaleC. opilio spermathecae have been studied by several researchers (Beninger et a1. 1988; Sainte-Harie 1993i Sainte-Marie and Lovrich 1994). Mating behaviour of C. opilio based on laboratory observations has been described by Watson (1972) ....hile field studies have been carried out by Taylor et al.
(198S), Hooper (1986), Ennis et a1. (1988, 1990) and Comeau eta1. (1993).
The primary objective of the present study is to investigate the impact of the commercial fishery on female sno.... crab reproductive biology in the ....aters surrounding insular Newfoundland. Previous comparisons of the proportion of mature females carrying eggs have revealed
Figure 1.3: Clutch of orange eggs attached to the pleopods of a mature female snow crab.
TlI;b1e 2.1. Relation of fecundity (Y , I eggs) to carapace width (X, nun) for snow crab Chionoecetes opilio from several study areas.
Study Area St!lomp1e size
ii fecundity Fecundity regression (I of eggs) Range
Newfound1emd· 51
NewfoundlandD ]50
Baie St!lointe-HargueriteG 195
Anticosti· 98
Pleasant Bay· 98
Gabal:'us· 115
Gulf of St. Lawrence· 99 NW Cape Breton Island4 25 Southeastern Bering Sea- 42
~from Davidson (l983) tofrom present study .. from SlI;!nte-Mar ie (1993)
4from Einer and Gass (1984) -from Haynes et al. (1976)
Yto6.4080XI,u, 52 , 048 37,934 - 81,2]9 Y • O. 7493X1.un 44,658 8 , 589 - 103,112
Y • O.7311X,
·1U 45,273 17 ,181 - 109 , 757 Y .. 13.2530)(1."21 58,760 12,134 - 122 , 891 Y • 38.4554)(1."" 74,475 32,564 - 128,433 Y '" 14.7361)(1 ..." 80,068 42 , 284 - 120,378 y • 0.0012X·· 1OO 20 , 000 - 140 , 000 Y=3092. 23XO,lQ 61 , 430 31,276 - 102,022 Y= 0.4905)(1.110' 36 , 273
11
little difference between regions that had varying levels of exploitation (EInar and Gass 1984). Data presented herein for the Newfoundland region are similar with the possible exception of periods When molting rates decline severely.
Potential effects on individual fecundity have not been previously investigated. In the present study, fecundity is compared among several areas experiencing varying degrees of commercial exploitation revealinghighlysignificant differences between exploited and virgin stocks. Differences among sites were also examined for trends relative to physical features such as depth and latitude. These differences were further examined to determine the relative effects of male size, age as indicated by female shell condition and recency of mating as indicated by spermatophore type. This analysis reveals a strong spermatophore affect indicating that mating recency significantly affects size-specific fecundity.
Finally, the efficacy of determining recency of mating from external observations of grasping marks is evaluated by comparison with internal spermathecal observations. This is a potential tool for rapid evaluation of the mating status of commercially exploited populations.
12 CHAPTER 2: Snow Cr.}) Fecundity
INTRODUC'l'ION
One of the principal methods used to ascertain the reproductive "health" of snow crab populations has been the monitoring of the proportion of mature females carrying eggs. Ifan exploited popUlation is undergoing stress in terms of fUlfilling its reproductive potential this could manifest itself as an increase of barren females. If barren females are extremely old-shelled then they could be classified as being reproductively "senile" (Elner and Beninger 1995). This should be considered a normal part of the life cycle and the numbers of such females in a popUlation at a given time ....ould be dependent on recruitment strength, natural mortality and possible discard mortality.
However, if a high percentage of barren females in an exploited population are relatively young, then it is possible that the removal of large males might have a negative impact on population fecundity. High percentages of barren females have been observed off Cape Breton Island (Elner and Robichaud 1986) and the Nearshore Avalon Peninsula area (Taylor and O'Keefe 1986).
While development of an index of the percentage of females carrying full clutches of eggs may provide an overview of the population reproductive "health" in relation
13
to exploitation rates and recruitment fluctuations, individual size-specific fecundity can be used to assess and individual fitness in terms of egg number or mass (Elner and Beninger 1995).
HE'llIODS
Time-series research cruises were carried out each year from1982-92 in three heavily exploited nearshore areas;
Bonavista Bay, conception Bay and Nearshore Avalon.
Specimens were obtained bybaited traps and females were separated from males and sampled separately. Maturity, carapacewidth, shell condition and size and developmental stage of the egg clutch were recorded for each individual female.
size-specific pecundity
specimens from some of these yearly, nearshore cruises and from opportunistic spring offshore cruises were examined to determine the reproductive status of females. A number of locations from areas with different exploitation rates and bathometric features were chosen in an attempt to obtain samples from as diverse a range of areas as possible.
samples were collected from the following snow crab management areas:
14 virgipAr.as
Much of the data collection for this study was conducted during a period of extensive areal fishery expansion as the crab fleet sought new fishing grounds to replace those where catch rates had fallen to unacceptably low levels. Two areas on the east coast of insular Newfoundland, Fogo Island and the Downing Basin. were identified as virgin grounds and samples were obtained shortly after fishinq activity was initiated. Fishing had always been prohibited in a third area, Bonne Bay, on the west coast of the Island until 1985 when an illegal fishery had decimated the standing stockotmature crabs (Ennis et a1. 1990). The information available indicates that this stock had never been previously fished. Specimens collected from this area were obtained prior to the inception of the illegal fishery.
The virgin areas canbedescribed as follows:
~ (Figure 2.1 (a)]: This area situated on the west coast of the Island of Newfoundland is a fjord in which there is a virgin popUlation of snow crab that has been studied extensively (Taylor et al. 1985), Hooper 1986, Ennis et al. 1988, 1990 and Comeau et al.
1994). Although commercial fishing only began in this management area (Area 44, Figure 1.2) in 1995,
15
~ G • ~
a
~52+- ..J..._~_ _'r-~-..J...-~--'----_+'52
50 50
48
46-6;+0---5T,8---,-5-6-~---T54---5r'2---+5046
Figure 2.1: sample sites where mature multiparous females were collected tor tecundity analysis: a) Bonne Bay, b) Fogo Island, c) Bonavista Bay, d) Conception Bay, e} Nearshore Avalon, t)Southern avalon, q)ottshore Avalon and h) Downing Basin.
1.
anecdotal evidence indicates that in the late fall of 1985 illegal fishing activity severely depleted the
Pogo 1:'I&p4 {Figure 2.1 (b)]: This area is located on the northeast coast of the island and was first fished in1984. The coltllRercial crab fishing qrounds are comprised of many isolated deep holes. While the area has been heavily exploited since i t was first fished, specimens examined herein were collected during the early summer of 1984 and are thus considered as coming from a virgin area.
Downing 'a,in (Figure 2.1 (h)]: This area was first fished (at a low level of effort) in 1981, approximately 9 months before the date samples were collected. For the purposes of this study, females collected during the spring of 1982 are considered as having come from virgin grounds. This area was rapidly fished out in 1982-83 and subsequently was virtually abandoned by the commercial crab fleet due to the long distance from shore and extremely low catch rates.
17 1Jrp1oited ke••
Many areas along the northeast coast of Newfoundland have been fished since the early 1970's. within this group of exploited areas there is vide variability in teos of exploitation rates and bottom substrate type.
The commercially exploited areas selected for this stUdy effectively represent the various types of fishing grounds prevalent at this time. The exploited areas frail Which samples for this study were collected are described as follows:
BO!1&vi,t, Bay [Fiqure 2.1 (c)l: The snow crab resource in this area has been SUbject to very heavy fishing pressure since the late 1970's. The commercial fishing grounds are located in a deep trough that runs more or less continuously from the head to the mouth of the bay.
conception Bay (Figure 2.1 (d)J: This area has been periodically SUbjected to extremely heavy fishing pressure and despite remedial action in terms of markedly red'.Iced quotas, CPUE recovery has been slow (Taylor and O'Keefe 1988). The fishing grounds in this area are restricted to a deep-water trough that through the centre of the bay.
,.
If.anbor. Avalop [Fiqure 2.1. (e)): In the early 1980's the snoW' crab resource in this area was so severely depleted that commercial activity virtually ceased
(Taylor et a1. 1994). This period. of reduced abundance lasted. from1982 until 1988. The commercial fishing grounds are qenerally restricted to depths in excessot 185mand are more or less continuous. A reduction in the percentage of berried females was detected in 1985 (Taylor and o' Keefe 1986), providing the stimulus for the present study.
SoutherA Ayalon [Figure 2.1 ( t l ] :The fishery in this area at the time specimens were collected was centered on commercial grounds that were quite restricted in area and therefore subject to intensive fishing activity and high exploitation rates.
Offshore Ava10g [Figure 2.1 (q)): This area, which was first exploited in 1981, experienced a collapse in resource abundance in the mid 1980's f0110winq the transfer of effort from the Nearshore Avalon. The commercial fishing grounds in this area consist of a series of three large "holes" progressively farther offshore that were all fished during the first year of commercial exploitation.
,.
specimen Collect.ion - Fecundity Analysis
This aspect of the study examined the reproductive status of 350mature berried females from several snoW' crab management areas around insular Newfoundland bet....een 1983 and 1985. only females bearing full clutches of bright orange eggs were retained for fecundity studies. Specimens were collected using standard Japanese-style conical crab traps baited with a mixture of northern short-finned squid (IllexillecebrosuS) and Atlantic mackerel (Scomber scombrus). Traps were fished in randomly selected positions stratifiedbydepth in 10n91ine fleets of 12. Weather permitting, fishing gear was hauled after a 24 h soak.
Eleven traps were fitted with standard commercial crab mesh
(~33mm stretch measure) and one trap per fleet was covered with small-meshed (25 mm stretch measure) webbing to obtain a sample of sub-legal sized crabs (including females) that normally escape through commercial mesh.
Upon haUling the traps, crabs were sorted according to sex. If the number of female snow crabs exceeded 25-30 individuals, then the catch was subsampled into two baskets.
Female crabs in one basket were counted and returned to the sea immediately; those in the other basket were retained as a representative biological sample. Samples were collected between April and July except for the Bonne Bay and conception Bay samples which were collected in October.
20
since sampling was conducted opportunistically during surveys directed primarily at male crabs, females were collected as they were encountered. In some areas all or most of the females used in this study were collected from one or two locations. In most areas however, they were collected in small batches or individually. Biological sampling entailed measuring the carapace width (ew) to the nearest1.0mm at its widest point, and classifying the molt stage as determinedbyshell condition according to the following criteria (Taylor et al. 1989):
1. "New-liIott" - recently molted crabs having an extremely soft shell bearing no epibiotic growth.
2. ,oHew-bard" - hard-shelled crabs having very little epibiotic gro....th on their shells. Crabs in this shell condition could be either primiparous or multiparous.
3. "Old-bard" - hard-shelled crabs carrying large amounts of epibiotic growth on their carapaces. Crabs in this shell category are considered multiparous.
Evidence of the recency of mating was collected by dissecting the animal and macroscopically determining whether the spermathecae contained old or new spermataphores
21
orboth (see Chapter 3). A representative size range of females carrying full broods of orange eggs was chosen randomly for fecundity studies. Egg.asses were removed intact with the pleopodsbysevering the abdomen from the body, andthen fixing the mass in 10\ fonnalio buffered with sea water.
Egg Count II and N•••
Fixed egg samples were drained, rinsed with distilled water and soaked in Gilson's fluid for 24 hours. Eggs were then stripped from the pleopods with fine forceps, carefully pressed through a 800 mm sieve in order to separate them from each other and captured on a 355mIllsieve. Each egg sample was placed in a pre-weighed aluminum boat, oven dried for 24 hat 6SOCand then total egg mass was detertllined to the nearest 0.1 mg. The total number of eggs for each clutch was estimated from counts of two small subsamplf!s weighing approximately 40 mg' and containing 500-700 eggs.
statistical ADaly.hi
The general allometric relationship between fecundity.
measured in terms of egg' number or total egg mass. and female size (measured as carapace width) was determined by log: lag regression. The additional effects of category
22
variables such as fishery status (virgin VB. exploited) capture location, spermatophere type (new vs. old), and female age (new vs. old shell) were investigatedbyanalysis of covariance using dummy variables in a multiple regression model (Nie 1975). Each variable was coded as a binary dummy variable, i.e. "I" ifa virgin area and "0" ifan exploited area, and then was tested in a mUltiple regression already containing logcarapacewidthto see i f i texplained additional significant variation in the elevation of the line (the intercept). The resultant model was of the general form:
log r =A+81D. +8 1D: •••• +Bs 109 CW
where F is the fecundity, CW is carapace width, A is the intercept, Bo are the regression coefficients and OJ are the dummy variables such as fishery status, sperm type, etc.
This model formulation has the advantage that the effect strengths of the category variables may be readily calculated by converting back to linear form. For example, the linear form of a model with only one dummy variable(Od representing fishery status (virgin - 1, exploited = 0) would be:
23
For an exploited area, 10',°1returns a value of 1 which has no effect on calculated fecundity (Le. it is the referenoe group) ...hile for a virgin area 10'.0. has the value of 10'1 . Ifthe regression coefficient81had a value of 0.0615 then the effect strength of the dummy variable "Virgin" wouldbe lOo.om= 1.152. This would indicate that the size-specific fecundity of females from virgin areas was on average 1.152 times that of females from exploited areas; in other words, 15.2%greater. Histograms and normal probability plots of the standardized residuals were examined at each step of model construction to check for departures from normality.
RESULTSAJfDDISCUSSION
Fecunditywas examined on two levels. Firstly, the percentage of females carrying eggs from time-series
",esearch cruises conducted in three heavily exploited commercial fishing areas was examined on a yearly basis from 1982-92. secondly, the individual fecundity in tenDS of egg numbers and egg mass of selected females was assessed for a number of areas with varying physical and biological factors and supporting varying levels of commercial exploitation.
2.
Percentaq_ of oviqeroua r . . . l ••
Asummaryotthe percentageat berried (egg-carrying) females for three C01llJDercial fishing areas from 1982-92 is presented in Table 2.2. With the exception of Nearshore Avalon in 1982 and 1983, when sample sizes were very low, the percentage of berried females ranged between 81-100t and usually exceeded 90t. There was an apparent decline in the percentage of berried females detected during annual spring research cruises in the Nearshore Avalon area between 1985 and 1987. The significance of this decline has been briefly discussed in Taylor and O'Keefe (1986) who noted that virtually all moltinq activity was curtailed during this period, possibly due to lower than usual water temperature
«-1.Joe). This cessation of molting activity precipitated a fishery collapsesinceno males recruited into the fishery for a number of years (Taylor et a1. 1994). In contrast, the percentage of berried females in Bonavista Bay (an extremely heavily exploited area) remained relatively stable and above the 90t level. Although molting activity in this bay was somewhat reduced during this time i t was s t i l l at a comparatively high level (Taylor et a1. 1994) as evidenced by the comparatively high initial catch rates experienced by the commercial fleet each year and by the high proportion of new-shelled legal-sized males captured during annual
25
Table 2.2: Summary of the percentage of mature females bearing eggs on the commercial fishing grounds of 3 snow crab, Chionoecetes opilio, management areas 1982-92.
YEAR BONAVISTA 8A Y CONCEPTION BAY NEARSHORE AVALON 1982 100 (N=86) 100 (N=74) 72.7 (N-U)
1983 100 (N-IJ95) JJ.J (N=6)
1984 97.8 (N-IOOS) 98.2 (N-IIO) 98.8 (N""172) 1985 97.5 (N-400) 98.5 (N"·S22) 944 (N-36) 1986 96.0 (N-29B) 100 (N-166) 92.3 (N-1716j [987 99.0 (N-394) 96.8 (N"758) 85.7 (N-224) 1988 100 (Ns286) 92.3 (N""I090) 92.9 (N=480) 1989 94.9 (N=472) SO.8 (NmJ96) 90.0 (N-240)
1990 99.7 (N-SSO) 97.9 (N-762)
199\ 97.9 (N""676) 98.1 (N=318) 99.2 (N-S24) 1992 100 (N=378) 100 (N=30) 100 (N-182)
26
research cruises (Taylor and O'Keefe 1986). Thus itseems that the effect of exploitation on the proportion of berried females is relatively weak except when aceo.paniedby extreme environmental conditions.
Individual Fecundity
The reduction in the abundance of commercial-sized and presumably sexually competent male snow crab provided an opportunity to determine whether the decline in abundance of large males in a population affected the fecundity and general "reproductive health" of females on an individual basis.
Size-specific raCllD4ity
Eggmassand number were determined for 350 female snow crabs from 8 management areas. Females ranged in size from 44-85lIJDcarapacewidth witha meanwidthof 65.81lllD..
Fecundity, in terms of the numberoteggs attached to the pleopods, ranged froID. 8,589 to 103,112 while overall mean fecundity irrespective of area was 44,658 eggs (Table 2.1).
In all study areas combined, individual fecundity was positively correlated with size, increasing exponentially with increasing carapace width (Figure 2.2). No between-year statistical differences were observed (ANOVA, p > 0.05) in the mean fecundity of temales from Bonavista
'""
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40 45 50 55
e
65ro
75 80 85 90CARAPACE WIDTH(mm)
Figure 2.2: Scatter plot and reqrcGoion of fccundJty (cqq numhol':") veroun carapaco wiclth fot' fcmalc5 [r:-om .111 nludv.11 "."1 •. ,"l1hi"...1
2.
Bay and nearshore Avalon Peninsula management areas therefore data for all years from these areas were combined for further analyses.
VirginVersus Exploited Areas
Analysis of covariance using multiple linear regression (see example in methods) analysis indicates that females from the three virgin management areas (Bonne Bay. Downing Basin and Fogo Island) were 15.2\ more fecund in terms of egg numbers/clutch than were same-sized females in commercially exploited areas (Table 2.3). Females from Fogo Island, demonstrated a smaller size-specific fecundity effect than those from the other virgin areas. This result may be attributable to the prior fishing history in that the commercial crab-fishing grounds had historically supported a heavy groundfish gillnet fishery with reported high levels of (large male) snow crab by-catch (Miller and Hoyles 1973).
The gillnet fishery for groundfish may have acted in a similar manner as a directed fishery in reducing the number of large males available for mating.
Fecundi ty in terms of total mass of the egg clutch was also examined in order to minimize variability associated with within-clutch differences in egg size and intra/inter area variability of egg size (Davidson et al. 1985; sainte- Marie 1993). Regression analysis of egg clutch mass
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against carapacewidthrevealed that the three virgin areas again had a higher size-specific fecundity for each size group than did commercially fished areas. In fact, fecundlty in terms of egg mass was 24.1% higher in virgin than in exploited areas. This difference in range between fecundity levels measured as egg numbers versus egg lIlass implies that egg size is correlated with egg mass. The observed correlationbet....een egg mass and egg size, calculated as egg mass/egg numberI was indeed highly significant ( r "-.463, p< .0001). This indicates that large egg masses generally contained smaller eggs.
The magnitude of fecundity effects varied widely among areas ranging from -13.6\for the commercially exploited Southern Avalon area to +20.5% for the virgin Downing Basin area (Table 2.3, Figure 2.3). In terms of egg mass as a measure of fecundity the effects ranged from -16.5 for the Southern Avalon to +36.U for the Downing Basin. This wide range of variation provides an opportunity to explore the effects of physical variables (latitUde and depth) and biological variables (male size, shell condition and spermatophore type) on size-specific fecundity.
31 100
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80
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.
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40 45 50 55 60 65 70 75 80 85 90
CARAPACE WIDTH(mm)
Figure 2.3: Scatter plot and regression of fecundity (egg number) versus carapace width for females from Downing Basin (solid figures) and Southern Avalon (open figures).
32 Pby.ical Variabl••
Latitud.
Jones and Si1lUllS (1983) demonstrated tilat latitudinal differences can influence fecundity in the mud crabHelice crassa in New Zealand. Fecundity, measured over a range of approximately 11. 5 degrees latitude, was highest in terms of egg number at the highest latitude, total egg mass was independent of latitude. There were no obvious latitudinal trends in the present data for Newfoundland snow crab (Table 2.4, Fiqure 2.4). However, effects could possibly be obscured by differences in the exploitation level of the areas surveyed. Examination of the virgin areas alone reveals a negative trendwithlatitude, contrary to expectation. Downing Basin had the highest fecundity effect in terms of both egg .ass and egg numbers whereas the two northerly sites (Fogo Island and Bonne Bay) had lower fecundity (Figure 2.4). Latitudinal range. (-3 degrees) sampled in this study was narrower than that of the New Zealand study and latitude effects could also be obscured by other sources of variation. It should be noted that although these results are contrary to the expected trend, only three virgin sites are represented.
Table 2.4:
33
List of fecundity sample sites with navigational coordinates.
SAMPLE LOCATION LATITUDE LONGITUDE
FOGO I5LAIfD 4' 55.6' 54 14.0'
BonE BAY 4' 33.1' 57 54."
BQllAVIS'l'A BAY 48 50.S' 53 22.5'
OFFSHORE AVALON 48 02.5' 51 24.9'
NEARSHORE AVALON 47 46.5' 52 11 7'
COIfCEPTIOR BAY 47 23.2' 52 56.8'
SOUTHERB' AVALON ·fA ·fA
DOWRING BASIN 47 02.3' 50 48.)'
SUMMARY OF EFFECTS OF LATITUDE IN RELATION TO EGG NUMBER AND EGG MASS
'"
CEGGNO.
CEGGMASS
DOWN.
BASIN
w
~ BOHNE
BAY
FOGO ISLAND l 3 0
~
::; 2<!§
§
~~ 10
'"
i:j I I I
l1. I I I , I I
~ ol!
0 I I ! I I I II I I I I I I I I t I I I~
II:.10
.2<!
LATITUDE RANKING (highest to lowest)
Figure 2.4: Size-specitic fecundity versus latitude.
35 Depth
Al though temperature has been observed to affect egg development time (Mallet et al. 1993) fecundity effects have not been reported. There are no seasonal temperature data for the collection areas reported herein but i t is expected that temperature should vary inversely with sampling depth which (except for the southern Avalon area) was recorded.
Mean depths of the areas sampled exhibited no obvious correlation with the fecundity differences observed among areas (Figure 2.5). The two areas with highest fecundity, Downing Basin and Bonne Bay. had mean depths that bracketed that of two of the lowest fecundity areas (Offshore Avalon and Nearshore Avalon).
The fact that fecundity does not appear to be related to depth does not rule. out the possibility of such an effect because the depth at which females were caught may not be representative of their average depth at the time that they were inCUbating eggs. Studies of long-term movements of male snow crab (Watson and Wells 1972; Taylor 1992) demonstrate that crabs can move more than 15 Janover a one year period. Other studies have documented seasonal changes in depth distribution for males (Miller and O'Keefe 1981) as well as for both sexes (Lovrich et a1. 1995). Sainte-Marie and Hazel (1992) also describe molting of immature male and female snow crab in shallow waters of the north....estern Gulf
.0 3.
30
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25z::>
~ 20
U -~~
UI!!
15~ ~
10w ..
5!w
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W
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-5-10
-15
SUMMARY OF EFFECTS OF BOnOM DEPTH IN RELATION TO EGG NUMBER AND EGG MASS
CEGGNO.
CEGO MASS
DEPTH 1m)
Figure 2.5: Size-specific fecundity versus depth.
u
'"
37
of St. Lawrence. Lovrich et al. (1995) document mature and immature males and females moving to comparatively shallow water in the winter and moving progressively deeper during spring and summer. The mating migration from deep to shallow water in the early spring has also been wel1- documented in Bonne Bay (Taylor at a1. 1985; Hooper 1986;
Ennis at al. 1988, 1990; Comeau at a1. 1991).
BiologicalDat.
XaleSize
The mean size of males captured coincidentally with the females (Table 2.3) showed no apparent trend with relative fecundity (Figure 2.6). Bonne Bay males were much larger on average than those from other areas (mean CW 116 mm), while those from Conception Bay and the Nearshore Avalon were much smaller. However, there ....as very little difference in the mean size of males from the other areas which displayed a wide range of fecundity effects from the highest (Downing Basin), to among the lo....est (Offshore Avalon) (Figure 2.6).
Thus, there was no obvious correlation between fecundity and the size of simultaneously captured males. The fact that no pattern was observed in these field data should again not be taken as evidence against the potential~of male size effects because the actual size of successfully mating males was unknown. It has been reported that large males
SUMMARY OF EFFECTS OF POTENTIAL MALE MATE SIZE IN RELATION TO EGG NUMBER AND EGG MASS
40
3.
~~ 30
lrl It
25 w~
20"
z&
15u~ 10
~
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~
w>i=
~
..
0:
-10
-1'
~
CARAPACE WIDTH lew rrml
Figure 2.6: Size-specific fecundity versus mean size of concomitantly captured male snowcrab.
39
have a higher rate of mating success than small males (Hooper 1986; Conan and Comeau 1986) although actual effects of male size on size-specific fecundity have yet to be documented.
Shell Condition - Aq. Effecta
Older multiparous females, as determined by shell condition (old-hard, see methods) had significantly lower fecundity than younger (new-hard) mUltiparous individuals (Table 2.5). Females with new shells were 10.5t and 9.0\
more fecund than old-shelled individuals in terms of egg numbers and egg mass respectively.
Previouisly, researchers (Haynes et al. 1976; Jewett 1981; Einer and Robichaud 1983) speculated that primiparous females might have a lower fecundity than mUltiparous female c. opilio. This ....as recently confirmed by sainte-Marie (1993) for specimens collected in the northern Gulf of St.
La....rence. He demonstrated that primiparous females were 16.4-22.7% less fecund that young multiparous females of the same size. These observations, combined with the age effect observed here, indicate that popUlation age structure can have significant effects on overall fecundity of snow crab popUlations. populations dominated by either young primiparous or old multiparous females should have lower
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41
fecundities than those with a more balanced age distribution.
Bperaatopbore Type
covariance analysis conducted on 253 individuals by spermathecal contents indicates that females carrying new spermatophores had higher fecundity than those with old spermatophores while females with both old and ne....
spermatophores were intermediate (Table 2.6). Females with either new spermatophores or new +old spermatophores were both significantly more fecund in terns of egg numbers than those with old spermatophores only. The analysis of fecundity in terms of egg mass revealed a significant difference between new and old spermatophores only. The size-specific fecundity difference between old and new spermatophores ..as 30.3\ for egg number and 21.0\ for egg mass (Table 2.6). The observation that size-specific fecundity tended tobehigher in females with new spermatophores than in those with both new+old spermatophores (although not significantly so: p= 0.11 for eggs, 0.18 for egg Illass) was unexpected because new spermatophores are at the base or the spermathecae and thus should be utilized first during fertilization.
42
Tab':'e 2.6: Pairwise comparison of sper:nacophore type affects on size-specific fecundicyi.,t.erms of egg number- (upper right section) and egg mass (lower left sect:ionJ. Ellen cell contains the effect coefficient: followedbyits pvalue, 8ample size and calculal:ed effect: sl:=enqth.
NEW !fEW ...OLD OLD
NEW 0.05151 0.11479
p=O.1060 p<O.OOOI N"'130 N=140
+12.6\ +30.3\
NEW ... OUI -0.04422 0.02945
p-O.178? D"'0.0402
N,.-130 N-236
-lO.n +7.0\
OLD -0.10231 -(/.02612
paO.COO2 p=O.0813
N-140 N""236
-21.0\ -5.8\
43
Joint Analysis of Sbell Type &114.p.~topbor. Type eftect.
Astepwise model of the joint influence of spermatophore type and shell condition revealed that shell condition did not explain significant additional variance in size-specific fecundity when added to models containing spermatophore effects (models 1 and J in Table 2.7). In contrast, spermatophore type explained significant additional variation in size-specific fecundity measured as eggnumber when added to a model that already contained shell condition (model 2 in Table 2.7) while the extra explained variancetor predicting egg mass was at the borderline of siqnificance (model 4 in Table 2.7).
Therefore the shell condition effect seems to be weaker than the spermatophore effect. The t ....o factors are partly confounded statistically because new-shelled females tend to have new spermatophores (r= 0.523).
The "virgin area" factor contained significant independent information additional to each of the above models (see "remaining variables" in models 1-4, Table 2.7).
This indicates that there are explanatory factors other than spermatophore type and shell condition that remain to be elucidated.
Table 2.7: Step....ise regression models of fec:..mdity versus carapace width and biological and site factors.
Depeqdegt variable : Log Egg Number MODEL 1
IndepeAdept: variables r' p
NewEjaculate -0 .0688 0.651 2.37 0.0184 OldEjaculate -0.0291 0.657 -2.06 0.0404
Constant -0.0975 -0.44 0.6607
Remainipg Varhbln
NewShell 0.96 0.3355
Virgin Area 2.88 0.0044
MODEL2
Ipdep,ndent Variables r' p
Log Carapace Width 2.5994 0.638 20.75 <0.0001
NewShell 0.0407 0.647 1.92 0.0563
Old Sperm -0.0313 0.654 -2.22 0.0275
Constant -0.1088 -0.48 0.6348
Remainigg Variables
NewEjaculate 1. 69 O.0922
VirginArea 3.09 0.0022
Table 2.7: (Continued) pepepdent Variables· Log Egg MI"
MODEL 3
Independent Variables r' p
Log Carapace Width 2.8211 0.661 22.36 <0.0001
NewEjaculate 0.0748 0.670 2.57 0.0105
Constant -0.7176
-,
13 0.0019Remaining variables
Old Ejaculate -1-75 0.0811
NewShell 0.86 0.3919
Virgin Area 3.35 0.0009
MODEL
•
Indepegdept variable, r'
Log Carapace Width 2.8256 0.661 21.78 <0.0001
NewShell 0.0427 0.667 1.99 0.0470
Constant -0.7268
Remaiping Variables
NewEjaCUlate 1.83 0.0683
Old Ejaculate -1.93 0.0544
Virgin Area 3.58 0.0004
46 SlJIOWlY
The results of the investigation into snow crab fecundity can be summarized as follows:
1. The observed percentage of berried females in snow crab populations usually was high (>90\") and was largely independent of the commercial exploitation rate.
2. Size-specific fecundity of females in virgin areas was higher than that of those in commercially exploited areas.
Asignificant portion of this higher fecundity can be attributed to spermatophore type with females possessing ne....
spermatophores significantly more fecund in terms of both egg number and egg mass. From this it can be inferred that recency of mating affects fecundity, i.e. mUltiparous females utilizing new spermatophores are more fecund than those relying on old spermatophores from a previous mating.
3. The enhanced fecundity bestowed by new spermatophores suggests that a rapid method of assessing recency of mating might be useful in comparing snow crab population reproductive status.
47
CBAP'l'EJl 3: Bz~.rD.lCl•• lIiticatioll of Kating Racency :IftRODUC'l':ION
The analysis of size-specific fecundity has revealed a strong effect related to spermatophore type (Chapter 2).
Females bearing only old sperm had significantly lower fecundity than those bearing new spermatophores. This suggests that recencyot mating should be correlated with individual fecundity and thus might be used as an indicator of popUlation reproductive status.
One way the male only commercial snow crab fishery might thus affect reproductive output of a popUlation would be through reduced frequency of mating possibly due to either reduced male density and/or size. There is strong competition among males both for primiparous (Sainte-Marie and Hazel 1992) and multiparous (Hooper 1986; Conan et a1.
1994) females. Larger sno.... crab males have been observed in the laboratory (Conan and Comeau 1986) and in situ (Hooper 1986) to be more effective than smaller males in securing and mating females. Some of the implications of depletion of large males have been discussed by Sainte-Marie and Lovrich (1994), Conan and Comeau (1992), Elner and Beninger (1995) and Ennis et a1. (1990). Heavy fishery exploitation ....i11 shift the popUlation from a relatively static state comprised for the most part of large, old-shelled morphometrically mature male and multiparous females to a
48
population dominatedbynew-shelled smaller individuals (Ennis et a1. 1990; Conan et a1. 1992). These expected changes in size distribution can be observed in Newfoundland snow crab populations. For example, the virgin population in Bonne Bay was dominated by large old-shelled males (FigureJ.1) while heavily exploited populations such as that in conception Bay had much smaller males with a smaller size range (Fiqure 3.1) (see other concomitant male size frequency distributions in Appendix 1).
Such large changes in male size distribution might affect the frequency of mating by females. This would in turn affect the proportion of mUltiparous females bearing new spermatophores ....hich has been shown to affect fecundity (Chapter 2). It would thus seem valuable to have a rapid means of assessing mating recency in natural popUlations so that potential fishery exploitation effects couldbe monitored. Frequency of mating could be determined for mUltiparous females by measuring the proportion carrying new spermatophores but this requires sacrificing the animals for internal examination which is often difficult to do under field operating conditions and can in any case be time- consuming. Therefore, i t would be useful if a more expedient and less destructive indicator of mating recency were available. Grasping marks left by males on hard- shelled mUltiparous females might be such an indicator.
.9
~ll&f1WZ5lI'O"'(lU.I oc:ru[lI un
n
Figure 3.1: Comparison of size-frequency distribution of concomitantly capture male snow crabs from a virqin area (Bonne Bay) and a heavily exploited area (Conception Bay).
50
During the precopulatory and copulatory mating embrace, the male holds onto and immobilizes its mate by grabbing her periopods ....ith his chelae (Watson 1972; Taylor et al. 1985;
Hooper 1986). primiparous females mate in the soft-shelled condition and their pliant exoskeleton does not usually bear grasping marks (Paul 1984), thus making i t extremely difficult to observe any external indicator of mating unless they are examined within a matter of days. In contrast, the mating embrace causes abrasions and scaring of the shell of hard-shelled mul tiparOlls females which leave detectable grasping marks. As the shell of the female ages, so do the grasping marks i t bears. Grasping marks which are initially bright-light coloured abrasions become dull and discoloured over time. Frequently they become infected by
chitonoclastic bacteria Which cause a slight necrosis of the shell tissue. turning the scars black.
Grasping marks have previously been used to assess the recency of the mating embrace during the annual mating migration in Bonne Bay (Taylor et al. 1985; Hooper 1986) and also in laboratory pairing experiments to evaluate the mating capability of small males (Ennis et al. 1988). The efficacy of using grasping marks as an indicator of mating success for females captured on the commercial fishing grounds is herein evaluated by comparing external
51
observations of grasping marks with internal examination of spermathecal contents.
NE'l'IIODS
During the period. 1987-92 inclusive, time-series cruises were conducted off the northeast portion of the Avalon Peninsula (Fiqure 1.2, Management Area 18). These cruises were carried out in the spring, when mature females would be best sui ted for testing whether the presence of fresh grasping marks provided a reliable indicator of recent mating. Mating status was determinedbyqualitative internal observations of the condition of the paired spermathecae to determine the age of spermataphores, and by qualitative external observations on the grasping marks on the ....alking legs.
8p.gu.tbeCA'Ob,.rvations
Macroscopic observations of the spermathecae were accomplished by simply removing the carapace and teasing out the spermathecae trom the visceral mass. Spermathecae were classified into the tollowing categories detined by the colour and texture of the spermathecal contents (Adams and Paul 1983; Paul 1984):
52
1. JfE1rONLY - Spermathecae having a very full appearance and containing only a bright-white, semi- solid matrix of spermataphores "capped"bya thin layer of brown gel (Figure 3.2).
2. OLD , HEW - Spermathecae consisting' of two distinct layers of ejaculate. The apical portion of the spermathecae containing a hard, yellow-ish to cream- coloured ejaculate "capped" byan opaque, dark brown or charcoal-coloured area and the distal section containing ejaculate consistent with the description for "new only" (Figure J. 3) .
J. OLD ONLY - Spermathecae containing only old, yellowish-beige sperrnatophores of a hard waxy consistency. Spermathecae appearing to be less full than those in the previous categories.
4. MOlfE - spermathecae empty.
5. trlfDOWlf - Spermathecae clearly not fitting any of the above categories.
53
Figure 3.2: spermathecae bearing new spermatophores only.
53
Figure3.2: Spermathecae bearing new spermatophores only.
55 Gr••pillq Mark Observations
Grasping marks were placed into four categories briefly described as follows and as illustrated in Figure 3.4:
1. NEW ONLY - Fresh, light coloured abrasions easily seen on the meri of the periopods of one or both sides of thebody. No evidence of blackening of the shell caused by bacterial invasion of the chitin.
2. OLD and NEW - New grasping marks overlying old, blackened grasping marks from a previous mating.
3. OLD ONLY - Grasping marks consisting solely of old, blackened abrasions.
4. UNKNOWN - Abrasions that could not be categorized withcertainty intothe above categories.
5. .ONE - No clear evidence of grasping marks.
Females classified in the "NONE" grasping mark group are most commonly primiparous. There might sometimes be an indentation on the periopods of these "NONE" category females suggesting that they had mated in the soft-shell
56
Figure 3.4: Legs of multiparous female depicting the three types of grasping marks. Uppermost leg bears new marks only, the middle leg bears old marks only while the lower leg bears both old and new marks.
57
condition but well-defined grasping marks similar to those observed on multiparous females are rarely present. Paul (1984) reported that primiparousC. bairdimated in the laboratory usually did not bear grasping marks. Females without grasping marks were considered primiparous in the present study ifthey were new-shelled. It is also possible for old-shelled females to produce a second clutch using stored spermatophores from their primiparous mating which would not have left grasping marks. A recent laboratory study (sainte-Marie and Carriere 1995) has demonstrated that this can occur quite frequently inC. opilia. Only females classified as having grasping marks on their periopods were included in the present study.
A small proportion of females with grasping marks did not have spermatophores in their spermathecae and these animals were excluded from the analysis because their recency of mating could not be determined. Soft-shelled females mated under both laboratory and field conditions have been observed to possess empty spermathecae despite bearing full egg clutches (personal observation). Paul (1984) found that the volume of spermatophores in the spermathecae shrinks with each successive fertilization of eggs and Sainte-Marie (1993) determined that after egg extrusion and fertilization the weight of the spermatophores within the spermathecae shrinks during storage. This fact
58
combined with the observance of empty spermathecae in grasping-marked multiparous females suggests that the full spermathecal contents were used up either through a reduction in seminal fluids as the spermathecal contents aged (sainte-Marie 1993) or through loss during fertilization.
RESULTSAJlDDISCUSSION Potential 8ia. Due to Int.eraction. Within Traps
The fact that trap-caught females have been confined for 24hours or more with males raises the possibility that new grasping marks might result from trap interactions rather than actual mating encounters. This was evaluatedby comparing trap-caught and trawl caught-females from offshore cruises (Figure 3.5) to seeif there wasilldifference in the percentage of females bearing grasping marks between gear types.
Four cruises were conducted on the LADY HAMMOnD between 1989 and 1992 during which mature females were examined to determine ....hether trap-caught tebales exhibited inflated percentagesotrecent grasping due to matinq attempts ....hile in the confines of the traps. Ovigerous females were captured bothby commercial and small-meshed crab traps and by a Western IIA otter trawl ....hieh should have reduced these interactions. Data for both collection methods were
59
-60 -58 -56 -54 -52 -50
YtT---L., ---7,----''---(-a).-90-'----~-+,~
48
46_
-60 ·58 -56 -52 -50
Figure 3.5: Location of offshore survey sites, 1989-92.
-NA" and large solid circle denote Nearshore Avalon time-series sampling sita. Lower case letters and small solid circles denote offshore sampling sites: a)FunkIsland, b) Offshore Avalon, c) Offshore Avalon, d} Downing Basin, e) Haddock Channel and t)Halibut Channel.
